Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame.
Special relativity suggests that the concept of simultaneity is not universal: according to the relativity of simultaneity, observers in different frames of reference can have different measurements of whether a given pair of events happened at the same time or at different times, with there being no physical basis for preferring one frame’s judgments over another’s. However, there are events that may be non-simultaneous in all frames of reference: when one event is within the light cone of another—its causal past or causal future—then observers in all frames of reference show that one event preceded the other. The causal past and causal future are consistent within all frames of reference, but any other time is “elsewhere”, and within it there is no present, past, or future. There is no physical basis for a set of events that represents the present.
If you get this, you can move on. If not, then let me try to explain this simply; with analogies and without jargon.
We can be separated by space. For example, I can be standing on the sand at the shore and you can be knee-deep in the sea. Say we agree to raise our hand up at the same time, and do so. It may seem that this event proves there is a now. There is a snapshot of time in which we both raise our hand up.
However, there exist ways for a third observer to see me raising my hand first, or you raising your hand first. This has nothing to do with the biomechanics of reaction speeds or cognitive illusions. It is possible for an observer to literally see and measure what are precisely two simultaneous events from our perspective to not be simultaneous. This is because we all exist on our own “reference frame.”
It is impossible to affirm that I universally raised my hand at the same time as you. We did or didn’t, both views can be physically correct.
For example, a princess falls asleep in Tokyo and another on the Moon. Imagine a being halfway between these two places that has godlike vision, it sees them give their last blink at the same time. However, if another being is flying from Tokyo towards the Moon, it will see the princess on the Moon doze off first. It does not make sense to ask, “But which one really happened?” The god-eyed being resting between Tokyo and the Moon could take a photo of the situation, and then later meet up to compare this with the photo taken by the other god-eyed being who had been soaring to the Moon, and they would have different photos. If they then compare their results with yet another frozen snapshot taken by a being who had been plunging from the Moon towards Tokyo, they would find evidence of another version of the events in which the Tokyo princess was already asleep while the other’s unmistakable aquamarine gaze was yet peering into the stars.
There is a way for the universe to line up the events so that all reference frames agree that one of the princesses fell asleep first. The universe does this by gathering up the fragments and connecting them on a strand of light. This is called causality, and this is how it’s done:
The Tokyo princess closes her eyes. Now quick, count, 1 tick, 2 tock, 3 tick, 4 tock, 5 tick, 6 tock, 7 tick, 8 tock, 9 tick, and the princess on the Moon closes her eyes. We have time 9 seconds. Now set aside this 9 we have collected for we will need to weigh it against another number. If the 9 we have collected is greater than the number we will collect, then we will succeed at preserving the order.
Now we must create the opponent. To transmute 9’s contender, we must take the distance 238,900 mi (from Tokyo to Moon) and divide by 186,282 miles per second (the speed of light)… And the opponent created from distance and light speed, measures in at 1.28.
Now weigh these and pray that our 9 is larger than this 1.28. Yes! 1.28 is definitely smaller than 9. We have succeeded at preserving the order! Now no one will have to disagree that the princess in Tokyo closed her eyes first.
Notice that the universe only succeeded because it didn’t see the princesses doze off at the same time. But what if it saw 0 time elapse between the shutting of each of the lady’s respective eyelids? This is what the god-eyed being resting half-way between the Moon and Tokyo saw. It is not what the being shooting head-first toward the Moon saw, and this is unavoidable. There are different reference frames. Shakespeare said, “All the world’s a stage.” He was wrong. You are wrapped in your stage as you move through the world.